This module provides the basic infrastructure for writing asynchronous socket
service clients and servers.

There are only two ways to have a program on a single processor do “more than
one thing at a time.” Multi-threaded programming is the simplest and most
popular way to do it, but there is another very different technique, that lets
you have nearly all the advantages of multi-threading, without actually using
multiple threads. It’s really only practical if your program is largely I/O
bound. If your program is processor bound, then pre-emptive scheduled threads
are probably what you really need. Network servers are rarely processor
bound, however.

If your operating system supports the select() system call in its I/O
library (and nearly all do), then you can use it to juggle multiple
communication channels at once; doing other work while your I/O is taking
place in the “background.” Although this strategy can seem strange and
complex, especially at first, it is in many ways easier to understand and
control than multi-threaded programming. The asyncore module solves
many of the difficult problems for you, making the task of building
sophisticated high-performance network servers and clients a snap. For
“conversational” applications and protocols the companion asynchat
module is invaluable.

The basic idea behind both modules is to create one or more network
channels, instances of class asyncore.dispatcher and
asynchat.async_chat. Creating the channels adds them to a global
map, used by the loop() function if you do not provide it with your own
map.

Once the initial channel(s) is(are) created, calling the loop() function
activates channel service, which continues until the last channel (including
any that have been added to the map during asynchronous service) is closed.

Enter a polling loop that terminates after count passes or all open
channels have been closed. All arguments are optional. The count
parameter defaults to None, resulting in the loop terminating only when all
channels have been closed. The timeout argument sets the timeout
parameter for the appropriate select() or poll() call, measured
in seconds; the default is 30 seconds. The use_poll parameter, if true,
indicates that poll() should be used in preference to select()
(the default is False).

The map parameter is a dictionary whose items are the channels to watch.
As channels are closed they are deleted from their map. If map is
omitted, a global map is used. Channels (instances of
asyncore.dispatcher, asynchat.async_chat and subclasses
thereof) can freely be mixed in the map.

The dispatcher class is a thin wrapper around a low-level socket
object. To make it more useful, it has a few methods for event-handling
which are called from the asynchronous loop. Otherwise, it can be treated
as a normal non-blocking socket object.

The firing of low-level events at certain times or in certain connection
states tells the asynchronous loop that certain higher-level events have
taken place. For example, if we have asked for a socket to connect to
another host, we know that the connection has been made when the socket
becomes writable for the first time (at this point you know that you may
write to it with the expectation of success). The implied higher-level
events are:

Event

Description

handle_connect()

Implied by the first read or write
event

handle_close()

Implied by a read event with no data
available

handle_accepted()

Implied by a read event on a listening
socket

During asynchronous processing, each mapped channel’s readable() and
writable() methods are used to determine whether the channel’s socket
should be added to the list of channels select()ed or
poll()ed for read and write events.

Thus, the set of channel events is larger than the basic socket events. The
full set of methods that can be overridden in your subclass follows:

Called on listening channels (passive openers) when a connection can be
established with a new remote endpoint that has issued a connect()
call for the local endpoint. Deprecated in version 3.2; use
handle_accepted() instead.

Called on listening channels (passive openers) when a connection has been
established with a new remote endpoint that has issued a connect()
call for the local endpoint. sock is a new socket object usable to
send and receive data on the connection, and addr is the address
bound to the socket on the other end of the connection.

Called each time around the asynchronous loop to determine whether a
channel’s socket should be added to the list on which read events can
occur. The default method simply returns True, indicating that by
default, all channels will be interested in read events.

Called each time around the asynchronous loop to determine whether a
channel’s socket should be added to the list on which write events can
occur. The default method simply returns True, indicating that by
default, all channels will be interested in write events.

In addition, each channel delegates or extends many of the socket methods.
Most of these are nearly identical to their socket partners.

Bind the socket to address. The socket must not already be bound. (The
format of address depends on the address family — refer to the
socket documentation for more information.) To mark
the socket as re-usable (setting the SO_REUSEADDR option), call
the dispatcher object’s set_reuse_addr() method.

Accept a connection. The socket must be bound to an address and listening
for connections. The return value can be either None or a pair
(conn,address) where conn is a new socket object usable to send
and receive data on the connection, and address is the address bound to
the socket on the other end of the connection.
When None is returned it means the connection didn’t take place, in
which case the server should just ignore this event and keep listening
for further incoming connections.

Close the socket. All future operations on the socket object will fail.
The remote end-point will receive no more data (after queued data is
flushed). Sockets are automatically closed when they are
garbage-collected.

A file_dispatcher takes a file descriptor or file object along
with an optional map argument and wraps it for use with the poll()
or loop() functions. If provided a file object or anything with a
fileno() method, that method will be called and passed to the
file_wrapper constructor. Availability: UNIX.

A file_wrapper takes an integer file descriptor and calls os.dup() to
duplicate the handle so that the original handle may be closed independently
of the file_wrapper. This class implements sufficient methods to emulate a
socket for use by the file_dispatcher class. Availability: UNIX.